1. Field of the Invention
The present invention relates to a technique of overlaying a plurality of image objects.
2. Description of the Related Art
Needs for high-resolution printing have grown along with the recent increase in the image quality of printers. A user gets various impressions of image quality depending on the properties of data and objects of print applications. In, for example, a text or graphic, edge portions have very high impact, and output at a high resolution is required. For a photo image or the like, importance tends to be attached to tonality rather than resolution. To meet this requirement, raster processing is recently used. Document data mainly containing a text is output at a high resolution to maintain the image quality. Document data such as a photo image is printed at a low resolution, thereby increasing the print speed.
However, when a text or graphic having a high resolution and a photo image having a low resolution are subjected to raster processing or rendering processing at different resolutions and overlaid directly, they are inadequately drawn.
To prevent this, Japanese Patent Laid-Open No. H10-243210 discloses an arrangement having two processing units for high-resolution processing and low-resolution processing. An area where an image object and a non-image object such as a text or graphic overlap each other undergoes low-resolution rendering processing that is used to render an image object. This processing is also applied to the non-image object, thereby overlaying the objects at a single resolution.
However, the above-described prior art has the following problems. First, since the rendering processing of each resolution is selected based on the attribute of an object, rendering processing of different resolutions cannot be performed for objects having the same attribute. For example, an image having higher priority on image quality such as an image to be used for design and a simple image for office applications have different characteristics even when the objects have the same “image” attribute. For this reason, a demand for changing the output resolution sometimes arises even when the attribute is the same. However, the conventional technique cannot deal with this case.
Additionally, if an object such as a text or graphic, which should be output at a high resolution, is converted into a low resolution for overlay processing, the image quality may be poor. However, overlay processing at a high resolution requires more resources such as a memory. The number of pixels to be subjected to the overlay processing also increases and affects the process speed. Hence, a method for efficiently implementing overlay processing at a higher resolution using little resources is demanded.
The present invention has been made in consideration of the above-described problems, and provides an image processing technique capable of speeding up image overlay processing at a low cost.